Polyalkoxymethylmelamines, especially polymethoxymethylmelamines, have been well known for their usefulness in serving as cross-linking agents in surface coatings and for treating paper and textile materials. See, for example, U.S. Pat. Nos. 2,715,619, 4,101,520; Canadian Pat. No. 773,170. Alkylation, especially methylation, of polymethylolmelamines improves the melamine-formaldehyde condensation products in their solobility characteristics, stability before application or curing, and efficiency in reacting with compounds containing active H atoms.
The alkylated polymethylolmelamines may be represented by the general formula EQU MF.sub.m R.sub.n
where
M=melamine PA1 F=combined formaldehyde, --CH.sub.2 O-- PA1 R=alkyl group with 1 to 4 C atoms, CH.sub.3 being the most important
When M is lower than 4 and n/m ratio is lower than 2/3 or 0.67, their preparation is relatively easy. For most of the applications for which they are used, however, the degree of polymerization of the melamine-formaldehyde condensates should be kept low, preferably monomeric. For this reason, the reaction conditions for their preparation must be so chosen as not to promote undue polymerization. For condensates with m higher than 4 and n/m ratio higher than 2/3, methylation becomes laborious. Since the alkylation or ether formation reaction is reversible, the water formed during alkylation tends to drive the reaction backward and reach an equilibrium. In order to obtain fully-methylated products, water must be substantially removed from the reaction mass. As a rule, a second methylation step is carried out to complete the reaction and obtain the desired product. Strongly acidic conditions are required to effect the alkylation; but to remove the water by distillation, the pH should be above neutral in order to prevent polymerization. Repeated acidification and neutralization, with the accompanying salt formation, render the operation inconvenient.
Usually, nitric acid or hydrochloric acid is used as the catalyst for alkylation. (See, e.g., British Plastics, Feb. 1943, p. 518; U.S. Pat. No. 3,322,762, 4,101,520, 4,293,692; Canadian Pat. No. 773,170.) These acids require very careful handling. Hydrochloric acid is very corrosive to the reaction vessels. Nitric acid, being oxidizing, is dangerous to use with the oxidizable compounds formaldehyde and methanol. Furthermore, these acids are not anhydrous. The water introduced with the acid is unfavorable to the alkylation reaction equilibrium.
A process for preparing methylated polymethylol melamines using an organic acid cation exchange resin as the catalyst has been disclosed (U.S. Pat. No. 3,488,350) without the usual acidification. It is, however, not very efficient. A large amount of the catalyst, more than 10% by weight of the total reactions mass, is required.